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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/esurf-2019-26
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/esurf-2019-26
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Jun 2019

Research article | 05 Jun 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Earth Surface Dynamics (ESurf).

How does the downstream boundary affect avulsion dynamics in a laboratory bifurcation?

Gerard Salter1, Vaughan R. Voller2, and Chris Paola1 Gerard Salter et al.
  • 1Department of Earth Sciences, University of Minnesota
  • 2Department of Civil, Enviornmental, and Geo-Engineering, University of Minnesota

Abstract. Bifurcations play a major role in the evolution of landscapes by controlling how fluxes such as water and sediment are partitioned in distributary and multi-thread channel networks. In this paper, we present the first experimental investigation on the effect of the downstream boundary on bifurcations. Our experiments in a fixed-wall Y-shaped flume consist of three phases: progradation, transitional, and bypass; the first two phases are depositional, whereas during the third, the sediment flux exiting the downstream boundary matches the input on average. We find that deposition qualitatively changes bifurcation dynamics; we observe frequent switching in the discharge partitioning under depositional conditions, whereas bypass results in long periods of time where one branch captures most of the flow. We compare our results with a previously developed model for the effect of deposition on bifurcation dynamics. The switching dynamics we observe are more irregular and complex than those predicted by the model. Furthermore, while we observe long periods of time where one branch dominates under bypass conditions, these are not permanent, unlike in the model. We propose that the range of switching timescales we observe arises from a complex interplay of downstream-controlled avulsion, and the effect of bars in the upstream-channel, including previously unrecognized long-timescale bar dynamics. Finally, we describe bifurcation experiments conducted with sand but no water. These experiments share the essential feedbacks of our fluvial bifurcation experiments, but do not include bars. In these experiments, we find that the sandpile grows symmetrically while it progrades, but bypass leads to one branch permanently capturing all avalanches. We conclude that the downstream control of deposition vs. bypass is likely a major influence on bifurcation dynamics across a range of physical systems, from river deltas to talus slopes.

Gerard Salter et al.
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Gerard Salter et al.
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Bifurcation Experiments: Timeseries, Images, Topographic Scans, St. Anthony Falls Laboratory, 2016-2017 G. Salter, V. R. Voller, and C. Paola http://hdl.handle.net/11299/202950

Gerard Salter et al.
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Short summary
Bifurcations are the switches that steer water and sediment in delta and multi-thread river networks, playing an important role in shaping the landscape. In lab experiments, we found that when the downstream branches grow through time, frequent switching in the water/sediment partitioning occurs. In contrast, once sediment freely exits the downstream boundary, long periods of time where one branch dominates occur; however, unlike our theoretical prediction, these are not permanent.
Bifurcations are the switches that steer water and sediment in delta and multi-thread river...
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